Appraising the use of environmental tracers to determine groundwater recharge rates: The impact of media anisotropy

In the past couple of decades various environmental tracers have been utilized by hydrologists to estimate long-term recharge rates to groundwater aquifers. In general, environmental tracers are readily identifiable constituents of atmospheric precipitation which move through geological media along with the percolating water. These constituents can either be part of the water molecules themselves (such as isotopes of hydrogen and oxygen) or they may be highly soluble, non-reactive solutes (such as chloride). Such constituents are referred to as environmental tracers because they are naturally occurring and because some near-surface process (either natural or anthropogenic) induces some quantifiable characteristic signature in the water. This characteristic signature effectively labels the water, and the label either remains unchanged or else it changes in a predictable fashion as that parcel of water passes through the subsurface environment. Assuming: (1) that the tracer is uniformly applied in space at the field scale, and (2) that one-dimensional vertical downward flow predominates, a variety of models have been developed to estimate average groundwater recharge rates through the unsaturated zone based on environmental tracer concentration versus depth profiles. 15 refs., 4 figs